Natural Channel Hydraulics and Sediment Transport

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Literature Review

The Federal Water Pollution Control Act

The Federal Water Pollution Control Act of 1972, or Clean Water Act (CWA), outlines the guidelines for the protection of surface waters in the United States (EPA, 2005).  This legislation provides the EPA with regulatory and non-regulatory means for improving and protecting the nation’s water quality.  The act provides funding for the development and improvement of wastewater treatment facilities and led to the development of programs to limit direct point source discharges to water bodies through the issuance of permits.  Beginning in the 1980s, the importance of reducing nonpoint pollution sources came to the fore.  Incentive and regulatory programs were created to help reduce pollution contributions from runoff and other nonpoint sources.  Section 303(d) of the CWA mandated each state develop a list of impaired waters which fail to meet state water quality criteria (EPA, 2002).  The CWA requires states to address impairments through the development of Total Maximum Daily Loads (TMDL).  A TMDL specifies the amount of pollution a water body can assimilate and still meet water quality standards (EPA, 2005).  The TMDL program was created as a broad approach to water quality improvement, including the entire watershed in the evaluation of impairments.  The first step in the TMDL process requires states to develop water quality standards (WQS) which must be approved by the EPA (EPA, 2002).  Sections 303(d) and 305(b) of the Clean Water Act require water bodies in the United States to be evaluated in the context of applicable WQS.  Water bodies in violation of state water quality standards are referred to as “impaired” and placed on the state’s 303(d) impaired waters list.  TMDL studies are required for all waters on the 303(d) list.  A source assessment is then performed to characterize pollutant loads within the watershed including permitted and non-permitted point sources and nonpoint source areas, each with loading rate estimates (EPA, 2005).  TMDL developers then estimate the required reductions using watershed and stream models to simulate purposed changes. The rise in urbanization, combined with past channel manipulation, has caused channel erosion prediction to become an increasingly important issue.  Urbanization increases storm runoff peaks, frequencies, and volumes, leading to channel downcutting and widening and increased sediment loads (Graf, 1975).  Sediment is the fourth leading cause of water quality impairment nationwide (EPA, 2005) and benthic impairments, often caused by excessive sedimentation, account for 11.5% of the total non-attaining river mileage listed in the Virginia DEQ 2002 303(d) impaired waters report (VADEQ, 2002).  Excessive erosion reduces water quality through increased turbidity and the transport of sediment-bound pollutants.  High sedimentation rates can alter the streambed, destroying the habitat of benthic macroinvertebrates and shading out emergent aquatic vegetation.  Sediment-bound nutrients can trigger algal blooms and damage aquatic ecosystems through eutrophication. Studies have shown that channel retreat rates may be as high as 1000 m/year, providing a significant portion of sediment loadings to streams (Thorne et al., 1997).  The contribution of channel erosion to overall sediment yields may be as high as 80% in urban watersheds (Simon & Thorne, 1996).  Correctly predicting sediment sources and magnitudes is essential in the development of land management strategies and Total Maximum Daily Load (TMDL) plans where sediment is identified as the cause of impairment.  Accurate models are needed to assess the impact of channel erosion on watershed sediment loads. Currently, the extensive input requirements and complexity of process-based watershed software capable of simulating channel erosion have prohibited software implementation in TMDL studies with impairments caused by sediment.  The time and money required for data collection and model development are unreasonable.  Many commonly used watershed and stream models such as GWLF, SWAT, and CONCEPTS have channel erosion subroutines imbedded within the overall software structure.  Little research has been done to verify the accuracy of such subroutines, thus limiting their application.  Research providing a more thorough understanding of the effects of model complexities on channel erosion predictions would allow TMDL developers and land managers to more effectively assess the impact of anthropogenic actions on watershed sediment loading.

Natural Channel Hydraulics and Sediment Transport

H. Graf (1984) realized the importance of sediment movement in natural systems, stating, “[t]he understanding and formulation of movement and transportation of solid granular particles in or through liquid bodies represent an important issue within the fields of hydraulics, fluvial geomorphology, and others.” A study of the processes involved in the movement of sediment within a river system is fundamental to understanding the form and behavior of stream channels. Knowledge of natural channel hydraulics is necessary for understanding sediment transport and fluvial geomorphology (Chang, 1988).  The characteristics of flow in river systems are the driving force for sediment movement and channel morphological transformations.  River systems are characterized as open-channel flow systems, bounded by a free-surface.  The shape of the free-surface must be established to allow the determination of hydraulic parameters needed for open-channel flow calculations.  This water surface profile is often determined using an energy balance approach.  Flow velocity, hydraulic radius, depth, roughness, and other hydraulic parameters may then be used to predict the magnitude of sediment transport processes. Assumptions are a necessary part of modeling natural channel flow systems.  Uniform, gradually varied, and rapidly varied flow are common classifications for open channel flow regimes.  The uniform flow assumption is valid in areas with constant physical properties along the flow path (Chang, 1988).  Man-made or significantly altered river channels may reasonably meet this assumption, but uniform flow is not valid for natural river channels due to the constant variation in channel shape and composition.  Uniform flow equations are, however, often applied to natural flow systems when the area simulated is an individual channel cross-section over whose length parameters can be said to remain constant. Changes in flow properties must be considered with respect to distance and time.  Systems are described as steady flow systems if physical properties can be said to remain constant over the time scale to which flow equations are being applied.  Most natural channel flow systems are considered unsteady: changes in discharge, velocity, and depth with respect to time.  The assumption of steady or unsteady flow is important in establishing the flow routing methods and equations to be applied to a river system. Flow adjacent to boundaries is considered to have three layers.  The laminar sublayer is a thin layer in contact with the boundary surface.  The transition layer encompasses the flow zone separated from the boundary layer only by the laminar region.  This zone is characterized by Reynolds numbers between five and 70.  The turbulent flow zone, most distant from the boundary layer, is characterized by Reynolds numbers greater than 70.  This zone exhibits a greater flow resistance due to eddy formation and the velocity distribution with depth is non-linear.  The rough boundary conditions of many natural channels have a roughness coefficient too great to allow the formation of a laminar sublayer. The basic assumptions in establishing the characteristics of natural channel hydraulics for modeling have a significant impact on estimating the forces associated with the flow regime.  The forces associated with moving water are best understood through a physics-based approach.  As previously mentioned, an energy balance approach allows fluvial geomorphologists and river engineers to estimate hydraulic parameters.  When modeling river systems assumptions and equations must be developed with this overall energy balance in mind. Three major characteristics of the water must be considered to understand the energy transfer throughout the river continuum; elevation, velocity, and discharge.  Elevation and slope are basic basin characteristics and are available in the form of topographic maps or Digital Elevation Models (DEMs). Discharge can then be found by multiplying the velocity predicted by Manning’s equation with the channel cross-sectional area determined through field surveys or analysis of regional geometry curves provided by the Natural Resource Conservation Service. River systems begin as headwater streams.  These streams are characterized by high potential energy, due to elevation, and high velocity (kinetic energy) associated with relatively steep channel slopes.  This energy is dissipated through the formation of a channel with an elevated roughness coefficient, creating high frictional losses.  Streams in the upper reaches of a watershed are often characterized by straight channels with pools and drops which create a system for maximizing energy dissipation.  In contrast, rivers in the lower parts of the watershed are characterized by decreased potential energy and shallower channel slopes.  The energy buildup in higher order rivers is not due to elevation or velocity, though center channel velocities may be significant, but by the increased discharge associated with flow accumulation. Stream power is a quantitative description of flow energy and may be used to develop estimates of sediment transport capacity. The total load carried by natural streams is broken down into three components, each with distinct transport mechanisms (Knighton, 1998).  The dissolved load component consists of particles which are transported in solution.  It is not considered a part of the sediment load.  Washload consists of fine particles, often cohesive, with diameters less than 0.062 mm and slow fall velocities.  It may account for as much as 95% of the total load in some river systems.  Once detached, washload particles remain in suspension, making the transport of this component commonly supply-dependent.  Washload particles are often detached as aggregates.  Strong cohesive forces between individual particles require very high shear stresses for detachment, making cracks between aggregates the most common failure surface (Knighton, 1998). Bed-material load is comprised of particles with a diameter greater than 0.062 mm.  This load component often consists of noncohesive sand and gravel particles.  Excess shear stress, though still important in predicting the initiation of motion, is less representative of sediment transport for this load component.  Though armouring or downcutting to bedrock may eliminate the bed material supply in some rivers, most systems are characterized by unconsolidated bed materials.  In these systems, the channel bed provides the supply, but particles in this class cannot remain entrained until periods of increased transport capacity, associated with elevated flows.  This condition of constant supply and limited transport due to high energy requirements for entrainment is known as a transport-limited system.  Particles may be detached but not transported due to the high stream power required for larger bed material transport.  Bed material transport is most responsible for the reshaping of stream channels. The bed-material load is further divided into bedload and suspended load.  The smaller bed-material particles are transported in suspension.  Larger particles are transported either by rolling, sliding, or saltation and constitute the bedload fraction. The combination of energy concepts and sediment transport mechanisms helps provide a better understanding of the formation and channel characteristics of streams.  Referring back to the discussion of the high energy environment of many headwater streams, it can now be deduced that this environment would consist of mainly large, transport-resistant sediments.  The small contributing areas feeding low order streams limit the supply of fine sediment from overland sources, and steep channel slopes allow them to readily transport fine sediment, creating an environment where little supply is available to contribute to the high demand for washload sediments.  For these reasons, headwater streams have bed sediments consisting mainly of gravels, cobbles, and boulders.  Any fine sediment with relatively slow fall velocities cannot be deposited in the highly turbulent flow. Conversely, higher order streams in the lower reaches of a watershed are provided a supply of washload sediments from increasingly larger upland contributing areas.  Fine sediment is supplied to these reaches by overland flow and transport from upstream sections.  Larger, capacity-limited particles from upstream reaches settle out more rapidly as you move downstream leaving only small bed-material and washload particles to contribute to the sediment load.  Washload particles remain in suspension while the fine bed-material particles fall out in pool sections where transport capacity is limited.

1.  Glossary
2.  Introduction
3.  Goal and Objectives
4.  Literature Review
4.1  The Federal Water Pollution Control Act.
4.2  Natural Channel Hydraulics and Sediment Transport
4.3  Terms for Channel Sediment Movement
4.4  Erosion and Sediment Transport Equations
4.5  Modeling Channel Degradation
4.6  Model Selection
4.7  Watershed Models
4.8  Reach-scale Models
4.9  Monitoring Channel Degradation
5. Methods
5.1  Watershed Description
5.2  GWLF Modeling Procedure
5.3  SWAT Modeling Procedure
5.4  CONCEPTS Modeling Procedure
5.5  Erosion Pin and Scour Chain Methodology
5.6  Statistical Analysis
6.  Results and Discussion
6.1  Monitoring Data
6.2  Sensitivity Analysis
6.3  Monthly Sediment Loading Estimates
7.  Conclusions
Modeling Channel Erosion at the Watershed Scale: A Comparison of GWLF, SWAT, and CONCEPTS

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